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In this activity, students build a photocell detector, and use it to detect different colors of light in a spectrum. Then they place the detector just outside the red region of the visible light spectrum and see that the detector detects the presence of light there, even though there is no color visible. Students learn that invisible light exists and that we can detect this light with instruments other than our eyes. In a final part of the activity, students investigate the infrared signals emitted by TV and VCR remote controls. The activities build upon each other and are best taught in order. Section 1 of the activity guide includes teacher notes, information on materials and preparation, student misconceptions and a student pre-test. Each activity section also includes teacher notes, student activity sheets, and answer keys. This activity requires some special materials (e.g. a small solar cell, alligator clip leads, plus common classroom materials (e.g., overhead or slide projector). This activity is the second of four activities in Active Astronomy, which are designed to complement instruction on the electromagnetic spectrum, focusing on infrared light.